1. Field of the Invention
This invention generally relates to thermal recording and particularly to a system for controlling the operation of a thermal recording head for recording image information on a recording medium. More in particular, the present invention relates to a thermal recording head driving control system for controlling the operation of each of a plurality of heat-producing elements arranged in the form of a linear array in a recording head, thereby controlling the level of heat produced by each of the heat-producing elements to allow to obtain a recorded image of excellent quality at all times.
2. Description of the Prior Art
In thermal recording, use is made of a thermal recording head provided with at least one heat-producing element such as an electrical resistor, whose activation is controlled in accordance with an image data whereby the resulting heat is used to record an image on a recording medium. In one form of such thermal recording, use is made of an inked ribbon which is placed sandwiched between the recording head and the recording medium, normally plain paper, so that ink is transferred to the recording medium when partly melted due to application of heat to form a recorded image thereon. In another form, instead of using an inked ribbon, thermosensitive paper is used as a recording medium and the heat produced by the recording head is directly applied to the paper to form a recorded image thereon. In such thermal recording techniques, the thermal recording head normally includes a linear array of heat-producing elements which are arranged side-by-side at a predetermined pitch and the recording medium is moved in the direction perpendicular to the longitudinal direction of the linear array. In this case, as is well-known for one skilled in the art, the longitudinal direction of the linear array is called the main scanning direction and the direction normal to the main scanning direction, which is the direction along which the recording medium advances, is called the auxiliary direction.
In such thermal recording, the density of an image recorded on a recording medium fluctuates depending upon various factors, among which the inter-activation time period, i.e., time period between two consecutive activations of a heat-producing element, becomes predominantly important if high-speed recording is desired, which is often the case. That is, when one of the heat-producing elements is selectively activated thereby being heated to or above a predetermined temperature level, a dot of image is recorded on the recording medium, and, then, the temperature of the thus heated element decreases exponentially to a base level, normally room temperature. Since it takes time for the heat-producing element to return to the base level, if the following activation of the same heat-producing element takes place too soon, then the recorded dot of image will be higher in density than the previously recorded dot of image, thereby causing fluctuations in image density. It is thus necessary to provide a sufficiently long waiting time period between the two successive recordings in order to avoid such density fluctuations, or to provide a means for causing the heat-producing element to cool down to the base level in an accelerated manner. In either case, the recording speed is rather limited.
Several proposals have been made to cope with the above-described problems, and they include Japanese Patent Laid-Open Publications, Nos. 55-142675 and 52-55831 and Japanese Patent Publication for Oppositions, No. 55-47980. However, none of them is satisfactory and there has been a need to develop an improved system for controlling the activation of heat-producing element in a thermal recording head.